1. Field of the Invention
This invention relates to a multi-lamp protection circuit for an electronic ballast. The multi-lamp protection circuit, which is realized with solid-state devices and hence can be of low cost and small size, is particularly used to isolate the abnormal lamp(s) from the normal lamp(s).
2. Description of the Prior Art
Electronic ballasts have gradually taken place of traditional heavy electromagnetic ballasts to drive fluorescent lamps because of their light, small size and better performances. In order to reduce the product cost, the electronic ballast is designed to drive multi-lamps when two or more lamps are installed in a same fixture.
At present, an electronic ballast mainly uses a half-bridge series-resonant-inverter to drive the fluorescent lamps at a high frequency. For such an electronic circuit, an excessive large current may flow through power devices of the inverter and the filament of the lamp, in case that the lamp fails to be started up or comes to its life end. Therefore, difficulties surely arise from how to get rid of abnormal lamps and how to maintain normal operation of lamps when a plurality of lamps are driven by a ballast. The present method of an electronic ballast with a lamp protection circuit is to use a common electronic ballast for a plurality of lamps for separating a damaged lamp with a relay. But using a relay with mechanical contacts not merely reduces reliability but has disadvantage of a large dimension, a loud noise, and low efficiency, forming an adverse consequence against demand for light small electronic ballasts.
The objective of the invention is to offer a multi-lamp protection circuit for an electronic ballast, which can be manufactured with less cost than the conventional ones, and possible to cut off the abnormal lamp(s) instantly to protect the ballast. As a result, the normal lamp(s) can be operated normally free from the outage disturbance.
The discriminate feature of the invention is that all lamp circuits are parallel-connected and energized by an inverter circuit, lamp current of every lamp is detected and referred to its abnormality-detect circuit, respectively. The control signal of each switch is AND gated by the output of each abnormality-detect circuit. Therefore, the inverter operation of the malfunctioning lamp may be suppressed by the abnormality-detect circuit output, while the other lamps will keep on operating normally. Thus, the multi-lamp ballast protection can be achieved at a reduced cost.